Thread: Galvanic corrosion.........

Hi,

I've had some experience with stainless steel fasteners and it really and I'm far from convinced that there worth it when clamping disimilar metals. Good for say new aluminium parts over say 3mm thick or stainless steel parts for example.

Remember using stainless steel bolts which high on the galvanic series and so will corrode the items your clamping, so for example if your clamping thinnish mild steel, that m.s. will corrode to protect the fasteners. So your protecting the bolt and making the part sacrificial.

Also the the problem is the corrosion be most probably under the head of the bolt or via the under the painted edge of fastener hole, so it could be be hard to see at a glance. Then you have to repair the part rather then replace the fastener.

You can separate the parts with rubber, thin fibre washers, and maybe some sorts of grease. You'll have to treat the inside of the hole edge too. This ok for trim items, but generally a no-no for items that transmit forces and it diminishes the bolt strength. SS bolts also may be unsuitable for items with high clamping preload, as this yield early compared to high strength steel.

Generally I'd go with electroplated zinc fasteners, inspect regularly and replace if any surface rust is visible anywhere on the fastener. If it is a really critical bolt assembly remove and check for necking corrosion under the head.

Any corrosion is is indicative of the sacrificial zinc coating having done its job and requiring replacement. Also a lot cheaper fastener then SS and easier to get replacements.

Clive

Thanks Clive,

Your thoughts are very much appreciated.

The jury is still out at the moment I must admit. I will be doing further research as the majority of nuts/bolts I am replacing are non-load bearing, rather just for the bulk head panels to floor panels etc.

Cheers,

When I rebuilt my SIIA I used a lot of stainless because when I stripped the car most of the original bolts were rusted solid. However, over the years I've realised that plated high-tensile bolts with Loctite or anti-seize on the threads - depending on the use - will work really well.

I have had some Birmabright body parts corrode a little where a stainless bolt was used, and where that area was exposed to a lot of salt water. I only use stainless now for accessories and little brackets and so on, but then these days I live in a nice dry place.

To expand on comments above. In marine equipment, where stainless is used with aluminium alloys, it is essential that the two metals be insulated from each other, otherwise, corrosion of the ally is quite rapid. Same applies to Landrovers, although the corrosion will be slower, unless you do beach work.

Most stainless steels are weaker than mild steels used for bolts and substantially weaker than high tensile bolts. Worse, most stainless steels work harden and are likely to develop cracks leading to early failure. Stainless steel should not be used for load bearing in areas subject to vibration or cyclic load (most places in a vehicle!) except where the circumstances and properties are well understood and allowed for.

The best type of fastener for bodywork on Landrovers is sherardised steel as fitted to earlier Landrovers (now virtually unobtainable), but galvanised (not zinc plated) fasteners are obtainable, although your fastener specialist may have to order them in specially and if this is the case will probably not sell small quantities - but they are not all that expensive in the small sizes we are talking about (6 x 25-50mm). The thick zinc coating on these lasts a long time, even in a marine environment, and protects the metal being clamped. Electroplated zinc, while cheap and readily available, is a poor second.

John

This problems been around since the series landy...

Dont put stainless anywhere near the thing. Electroplated nuts and bolts (available in up to grade 12.9) for load bearing stuff and galvanised stuff or ally rivets for things that aren't is the way to go. Even then A good coating of grease/paint is going to go a long way to helping solve the problem.

It sounds (or reads) as though they aren't meant to be together!

If this is the case then, why are kits of stainless nuts and bolts being put together and now being sold and the like?

Then you consider the other more structured parts parts like hinges for doors and bonnets?

They do look the goods!

Originally Posted by redneb72

They do look the goods!

pretty much that reason.

I have a whole diatribe I can throw down range about Function before form thats a principle thats long lost in todays throw away society of I dont want to learn how to do it or do it for my self, cant I make something else do it for me, make it someone elses problem OR pay someone to deal with it for me?

As JDNSW said, HT coated bolts can be hard to come by. Point in case, I just picked up some bolts to fit a Load leveller to the cross member M10x180 8.8 and Panhard rod bolts, M14x90 8.8. The original LR panhard bolts look to be Gal AND their head/nut size is larger than standard M14.....All I could get was black phosphate coated

Now M14 is not a common size to begin with, and to by a box of 50/100 min will not be cheap....

other bolts that can be a bugger to get hold of are the double hex head M10's that hold the swivel to axle flange.

Where are people buying the CORRECT HT fastners from, and are they GAL, Zinc or Black phosphate??

If LR use a bolt with a larger head than standard, am I to asume, in a structural application, that the larger head is required for clamping pressure etc?